1. Field of the Invention
The invention relates to a color separation technology as pertains to print processing, and in particular, to a spectral color separation technology for contiguously separating a color of a color image on a spectral basis.
2. Description of the Related Art
There are four of what are referred to as coloring materials, known as Cyan (C), Magenta (M), Yellow (Y), and Black (K), that are employed when printing a color image via a printer or other image forming apparatus. In recent times, additional coloring materials are also employed, such as a Light Cyan (Lc), a Light Magenta (Lm), and Gray (Gy), wherein a concentration of a particular color is reduced. On the other hand, Red (R), Green (G), and Blue (B) are each referred to as a spot color.
In a typical circumstance, a color space of an inputted image differs from a color space of an image that is printed using the aforementioned coloring materials. It is therefore necessary to convert the color space of the image data to a color space of the coloring material, in order that a color appearance of a printed image is made to conform to a color appearance of an inputted image. If, for example, the color space of the inputted image is an RGB space, and the color space of the printed image is a CMYK color space, it will be necessary to convert RGB values to CMYK values, a process that is referred to hereinafter as color separation. In such a circumstance, reproduction of a graininess and a tone characteristic of the inputted image, as well as the color thereof, is desired in the printed image.
A technology known as calorimetric color reproduction is typically employed as a method of reproducing the color of the image. Colorimetric color reproduction is a color reproduction method that uses a human visual sensory property, hereinafter referred to as metamerism, of identifying two substances as being of the same color even if their spectral reflectance characteristics differ, provided that their tristimulus values are equal when exposed to a given light source.
Following is a detailed description of the calorimetric color reproduction method.
FIG. 20 illustrates the calorimetric color reproduction method.
As shown in FIG. 20, a light that is emitted from a light source 2000 is incident upon a substance 2001, and a reflected light therefrom is incident upon a human eye 2002, whereby a shape or a color of the substance 2001 is recognized. The incident light is converted into a color signal by three types of a cell, known as a cone that is found in a retina of the human eye. A human being identifies a color based on an intensity of the color signal, known as the tristimulus values XYZ, which may be depicted using a set of equations (1) that follow, when a spectral distribution of the light emission source is defined as S(λ), the spectral reflectance of the substance defined as o(λ), and a spectral property of the human eye is an isochromatic function x(λ), y(λ), z(λ) as defined by the International Commission on Illumination (CIE):
                              k          =                      100                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                s                  ⁡                                      (                    λ                    )                                                  ⁢                                                      y                    _                                    ⁡                                      (                    λ                    )                                                                                      ⁢                                  ⁢                  X          =                      k            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                s                  ⁡                                      (                    λ                    )                                                  ⁢                                  o                  ⁡                                      (                    λ                    )                                                  ⁢                                                      x                    _                                    ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  Y          =                      k            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                s                  ⁡                                      (                    λ                    )                                                  ⁢                                  o                  ⁡                                      (                    λ                    )                                                  ⁢                                                      y                    _                                    ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ⁢                                  ⁢                  Z          =                      k            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                s                  ⁡                                      (                    λ                    )                                                  ⁢                                  o                  ⁡                                      (                    λ                    )                                                  ⁢                                                      z                    _                                    ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                                        (        1        )            
Where k is a normalizing coefficient. That is to say, the calorimetric color reproduction method uses the fact that an original color and a reproduced color are identified as being the same color if the XYZ values of the original color and the XYZ values of the reproduced color, as calculated using the equation (1), are congruent.
A variety of image processing methods are proposed as pertains to the reproduction of the graininess or the tone characteristic based on the calorimetric color reproduction method. A technology exists that employs dark and light coloring materials, with respectively differing intensities, to improve the graininess. See, Japanese Patent Laid Open No. 2000-343731, or U.S. Pat. No. 6,592,212.
A technology also exists that contiguously changes an overall quantity used of each respective coloring material for each respective combination of a plurality of types of the coloring material quantity to improve the tone characteristics. See, Japanese Patent Laid Open No. 2004-058622, or U.S. Patent Laid Open No. 2004/070777. T. Ogasahara and N. Ohta, “Verification of the Optimum Prediction Model” discloses employing among other technologies, an established technology known as a Celler Yule-Nielsen Modified Spectral Neugebauer for a predictive model of an estimation of the spectral reflectance of the printed material or other matter.
In the calorimetric color reproduction method, as depicted by the equation (1), however, the XYZ values are dependent upon the light emission source S(λ). Consequently, even if the XYZ values of the original color and the XYZ values of the reproduced color are the same under a given light source, it is by no means certain that the XYZ values of the original color and the XYZ values of the reproduced color will be the same under a different light source. The colorimetric color reproduction method guarantees a precision in the color reproduction only under a particular light emission source. In addition, a spectral sensitivity property x(λ), y(λ), z(λ) of the human eye is an average spectral sensitivity property, meaning that a color will not necessarily be perceived as the same color by all people, even if their tristimulus values are equal.
For the foregoing reasons, if the coloring materials with varying intensities, such as C and Lc, M and Lm, C and M, or LC and M, for example, possess their own respective different isochromatic property, the coloring materials will appear to be different respective colors depending on a change in the light emission source or a characteristic of a particular observer, no matter how much, as per Japanese Patent Laid Open No. 2000-343731, the graininess is improved. Consequently, graininess may end up being degraded in some instances. Also, in some instances, the change in the light emission source or the characteristic of the particular observer may result in degraded tone characteristics, giving rise to a pseudo contour as a consequence, no matter how much, as per Japanese Patent Laid Open No. 2004-058622, the overall coloring material quantity is changed contiguously.